In both military and civilian terrain mapping and object tracking there exists a need to enable coverage of an earth-fixed azimuth sector from high-altitude airships whose orientation continuously changes. The high-altitude airships are generally gas filled dirigibles or blimps that have shapes designed for maximizing their aerodynamic performance such as lift, maneuverability and stationary or forward movements. The airship's distinctive skin materials and craft shape often challenge equipment designers in their efforts to effectively mount information gathering instrumentation, such as radar systems. Still, high-altitude airships are receiving increased attention for use as radar sensor platforms because of the inherent capability of an unobstructed view of large segments of the earth's surface as well the large volume of available space within and/or around the airship.
Information gathering missions tend to require radar coverage over a broad azimuth sector that is fixed with respect to the earth's surface. However, various factors such as the airship's need to face into the wind, the variable direction of high altitude winds, and the airship's need to maintain a minimum airspeed for waste heat convection, forces airship orientation to constantly change with respect to the desired coverage sector. These factors require radar systems that can adapt to the changing attitudes in pitch, elevation, yaw and roll movements.
As a result, such high altitude airship radar sensors should not only be capable of providing coverage over the desired sector width, but should also be capable of continually reorienting the position of this sector coverage with respect to the airship. Consequently, radar orientation with respect to the airship provides few satisfactory options.
One option illustrated in FIG. 1a is to mount a planar phased-array radar flat antenna 110 inside an airship 102, such that it maintains coverage in a fixed direction by slowly rotating with respect to the airship as the airship orientation changes with respect to the earth. In this configuration, array normal is approximately centered in the desired coverage sector. Electronic steering is then used to position the beam within the sector. Such an internal planar phased array as shown in FIG. 1a provides a beam output that is restricted to about sixty degrees (60°) relative to array normal. Disadvantages associated with such an approach include the undesirable requirements for heavy mechanical components, including a rotary joint and coupler that are incompatible with lightweight airship applications. Furthermore, such a solution would require an increased propulsion power to compensate for a rotating radar antenna's angular momentum. Still further, the aforementioned planar phased array cannot provide instantaneous coverage over 360°. Moreover, such a solution would suffer significant beamsteering gain loss (e.g. >9 dB) near coverage limits, thus, severely compromising overall operational performance.
Another option illustrated in FIG. 1b is to install a non-planar radar antenna phased array 110 on an airship's doubly-curved surface as opposed to internally to airship 102 (see FIG. 1a), such that the phased-array conforms to a large fraction of the airship's outer surface 105. In such a surface-conformal phased array radar system, a portion of the array whose normal approximately matches the center of the desired coverage sector is activated and then used to form and electronically position the beam within the desired sector. Numerous problems exist with this approach as well.
As is known in the art, a collimated beam of radio frequency energy may be formed and steered by controlling the phase of the energy radiated from each one of a plurality of antenna elements in an array thereof. A portion of the array whose normal approximately matches the center of the coverage sector might then be activated and used to form and electronically position the beam within a geographic sector.
For example, the curvature of surface 105 varies as a function of position on the airship surface (which is made larger or smaller due to gas expansion and contraction) so that antenna radiator element-to-element separations must also change as a function of position in order to maintain conformality. In addition, non-uniform element-to-element separations degrade the shape, gain, and sidelobes of the electronically scanned beam. Furthermore, range coverage and azimuth beamwidth are non-uniform in azimuth, as the projected aperture changes significantly as a function of azimuth. Accurate beamforming and shaping is therefore difficult because the airship surface expands and contracts significantly due to air density and temperature variations and tends to undulate or flutter due to airflow.
Still further, manufacturing and construction costs associated with the above approaches are high, due at least in part because the variable surface curvature requires the sub-panels constituting the array be of many different shapes and designs, creating adherence problems analogous to the well publicized space shuttle tiling problem.
Time-varying aperture shape associated with the conformal array approach also causes pulse-to-pulse variations that limit clutter cancellation. Other problems associated with the aforementioned approaches include complicated power and signal distribution, as different parts of the array may be hundreds of meters apart. Changing airship shapes also make calibration difficult, particularly with regard to the difficulty or inability to inject test signals into the antenna elements in the above surface-conformal approach.
An alternative mechanism for a radar system useful in a vessel such as a high altitude airship, and which overcomes one or more of the above-identified problems, is highly desired.